A356 Preparation Process for Anodizing

A discussion started in
2008 but continuing through 2018

January 6, 2008

Q. Hi, I am currently working for a company that sells high voltage cable clamps that are sand cast (die casting will be considered) of the A356 alloy. For most customers this alloy provides sufficient corrosion resistance. However, we do receive some requests for an anodized clamp. I know this alloy is difficult to anodize and am therefore putting much thought into preparing a process that will result in a good quality anodize. Most of the knowledge I have obtained is from an old ASM metals handbook (8th ed.) and is likely out of date.
Please advise me on any parts of my plan that should be altered. Clearly I have not yet decided on most of the chemicals to be used. I have left out rinsing.

I would also like to know how important the mechanical finish is, must the surface be smooth to anodize well or just clean? In my mind it seems a good anodize requires a clean surface with minimal precipitate and small grain size, is this correct? Also hould I be considering grain refiners?

Thank you very much, any help is greatly appreciated.

James Endersbeeproduct designer - Melbourne, Victoria, Australia

January 9, 2008

Q. After more searching through the earlier posts on this site I have made a little more progress on the plan:

My theory is this:
Trichloroethylene removes all oils. Acid clean removes soils and oxide. Caustic etch evens out the surface but reveals more silicon. HF/nitric acid removes silicon. Surface is then clean and ready to anodize.

I removed the tumbling step as I am not sure it is necessary.

I am still very uncertain and am only stating my plan as a base to work with.

Thank you for any suggestions.

James Endersbee[returning]- Melbourne, Victoria, Australia

January 10, 2008

A. Step 1 is fine, but you may find the use of trichloroethylene regulated to an extent that an ultrasonic cleaning station may be an easier approach. Step 2 will usually be a non-etching alkaline cleaner rather than an acid cleaner. Don't forget Step 6, sealing -- it's not optional. Good luck.

Ted Mooney, P.E.finishing.comPine Beach, New Jersey

January 10, 2008

A. Aluminum fines can cause a serious problem in a vapor degreaser using trike. Notice it is "can" , not "will". There are some conditions that go with it, but many people do not properly maintain their degreasers. Check your literature on the degreaser and possibly the internet.

James Watts- Navarre, Florida

January 10, 2008

Q. Ok. Thank you for your help. I hate to ask questions that are too simple to be of any interest to the people on this site, but I have been researching this for over a week and still have not found a pretreatment process for these casts.

Please tell me for my understanding; is the purpose of the alkaline clean to remove oxides so the etch will be even?

Also I have heard many people suggest ammonium bifluoride as an alternative to the dangerous HF/nitric mixture.After researching ammonium bifluoride I have gathered that this substance separates into ammonium fluoride and HF and is not much safer?

James Endersbee[returning]- Melbourne, Victoria, Australia

January 11, 2008

A. The alkaline cleaner is intended to be a cleaner not an oxide remover; it is the nature of how cleaning works that alkaline cleaners are better than acid (higher temperature for greater solubility, caustic for saponification, etc.)

Ammonium bifluoride is slightly safer than HF, but still a scary process.

If you are missing anything here, it's that most anodizers buy proprietary cleaners, desmutters and seals, and the chemical houses that supply them are usually willing to run sample parts for you. Detailing a successful process is easier and faster when a skilled practitioner is working at it in a well equipped lab, especially if you are working with a difficult to anodize part.

Ted Mooney, P.E.finishing.comPine Beach, New Jersey

January 11, 2008

A. Stay out of alkali etch, it does the opposite of what you want. You want to decrease the area of silicon on the surface and increase the area of aluminum (which is what you "anodize"). Alkali etching chews away the aluminum and increases the area of silicon, you do not want to do that.

Clean in a non-etch cleaner to remove organic soil.
Treat in 100% Nitric Acid + 2 pounds per gallon ammonium bifluoride until the part turns "frothy white" all over, about 20 to 30 seconds, hurriedly rinse two times and anodize.

A. You are adding the ammonium bifluoride to get the fluoride ion. ammonium bifluoride is extremely safe to handle compared to many other materials if the barest of minimum safety procedures are used. HF on the other hand, even 48% is mean wicked and nasty, but is a relative pussycat compared to 70% HF.
What you end up with is a nitric acid/HF solution that is minorly worse than a plain nitric acid solution. If you use procedures for safely handling the nitric, you will be fine with the mixed acid. Again, HF acid, by itself is NASTY!

James Watts- Navarre, Florida

January 15, 2008

Q. Thank you all for your help.

The clamp is composed of 2 parts, top and bottom, with a bolt on each side to pull each part together and clamp onto the cables.

The strange result from the anodize is that each part turns out a different color. The bottom part turns out with little color change and the anodize is more easily scratched off. The top part turns out darker gray, which I understand is due to the silicon, but the anodize is harder to scratch off. The bottom part also appears to have been polished with a coarse belt on some surfaces (~20% of total surface area).

The shop has not opened after the new year yet so I cannot ask them their process.
Is it likely that while they polished the bottom part mechanically they instead used a stronger etch on the top part, revealing more silicon leading to the darker appearance? I am confused that if this is the case, the part with greater silicon surface content seems to have anodized better (harder to scratch off).

Robert, do I not need the caustic etch to smooth the surface? It is quite rough after sand casting.
Should I used vibratory tumbling (or some other method) to smooth it? I am still unclear whether a smooth surface is required for a good anodize. Also, does the acid treatment remove oxide?

Now, regarding HF/Nitric vs Ammonium Bifluoride. If ammonium bifluoride is the safer option, does this mean HF/nitric is the more effective option?
I think that I'm being told that HF/nitric is only slightly more dangerous but pure HF is extremely dangerous (Ted and James?).

Anyway, thanks again for all the help.

James Endersbee[returning]- Melbourne, Victoria, Australia

January 17, 2008

A. You are adding the bifluoride to nitric acid to get an equivalent (virtually identical) acid to a nitric-HF tank.

Caustic etch on a sand casting is a mixed bag as some of the high spots will be reduced and some will be accentuated.
Sand cast parts are typically not cosmetically attractive, especially when dyed.

I always thought that a high silicon aluminum needed to go thru a fluoride containing bath step. It is a decent combination of desmut/deoxidizer and etch when the tank is new.

James Watts- Navarre, Florida

January 21, 2008

A. James Endersbee,
Do exactly as Robert Probert suggested. I don't think you need to etch in an alkaline cleaner before or after it. But you can customize the process if it suits your needs.

Some years ago, I tried the same formula on sand casting and it worked.
If it works for you then let us (India) equal the series with Australia in Adelaide...Good luck

Hemant Kumar- Florida, USA

January 22, 2008

A. Now that I rethink it, Mssrs. Probert and Bansal are correct, of course. There is no advantage and a lot of disadvantage to alkaline etching in this case.

Thank you all. Hopefully I will have some samples done soon, in which case I will let you know how they turn out.

Hemant Kumar, while I appreciate your help, unfortunately I don't have the power to affect the outcome of the test! Let's see how it unfolds.

Can I not skip the ultrasonic clean? Isn't the alkaline clean effective enough?

James Endersbee[returning]- Melbourne, Victoria, Australia

February 7, 2008

A. First the generic answer, James: Unlike many other materials, aluminum cannot be electrocleaned; further, the alkaline cleaners for aluminum must be kept relatively mild. So it is often necessary to preclean in a vapor degreaser or ultrasonic cleaner. But, yes, sometimes a corner can be cut.

Now for the specific situation. None of us have seen the parts nor do we have a clear understanding of all of the fine details of the parts and the soils they have been exposed to. All of what you read on this thread -- I'm pretty sure you understand this, but we'll say it for the benefit of others -- is general guidance regarding what to try in your lab development phase. Nobody is suggesting that anyone design & install an anodizing sequence based on book knowledge of what sometimes works for someone else's parts. Good luck.

Ted Mooney, P.E.finishing.comPine Beach, New Jersey

April 19, 2013

Q. What is the balanced equation for adding ammonium bifluoride to nitric acid and water?

Chuck Ritchard- Grand Rapids, Michigan, USA

April 23, 2013

A. Hi Chuck. I don't think 'balanced equation' is the right term. I think you are asking in what ratio these chemicals should be mixed. I believe it depends on the application but that Bob Probert has offered one good answer. Good luck.

Regards,

Ted Mooney, P.E.finishing.comPine Beach, New Jersey

April 24, 2013

Q. My question regarding the balanced equation was to understand what the by-products were when mixing Ammonium Bifluoride and Nitric Acid and whether or not they could be reclaimed through a filtration process.

Charles Ritchard[returning]- Grand Rapids, Michigan, USA

April 29, 2013

A. Those materials do not react with each other to produce any filterable precipitates. They both just stay in solution.

Ted Mooney, P.E.finishing.comPine Beach, New Jersey

Analysis method for nitric acid and ammonium bifluoride for etch bath

June 3, 2018

Q. Hello,
Could you please recommend an analysis method for determining the concentrations of these two materials, nitric acid and ammonium bifluoride used for the etching of A356?
I would need this for maintenance of the bath.
I would like to double check with you "100% Nitric Acid + 2 pounds per gallon ammonium bifluoride"?
My lab chemist was a bit alarmed from the quantities.

Gabriel Even- Beit Nechemia Israel

June 5, 2018

A. Hello Gabriel,
A great place to look is the tech sheet for a product called MacDermid-Enthone Metex Etch Salts, prod #13051.
It's an ammonium bifluoride product that is intended to be used with or without Nitric acid, depending on your application, and it has analysis instructions for both (though the calculation for 'Etch Salts" may or may not be EXACTLY equivalent to ammonium bifluoride content).
I have an ancient wrinkly fax in my files and trying to find a link online I'm discovering that you need an account/pw to view...
I have my own minor tweaks to the methods; this is not *quite* what theirs is, but here ya go:
Here's the one for the ammonium bifluoride, and take it with a grain of salt (no pun intended!):
Start with 1mL bath solution (let any particulate settle before taking your aliquot).
Add a drop of phenolpthalein, and a splash of deionized water, then very carefully raise the pH to JUST pink (I use NaOH .1N for this so as not to overshoot) while swirling, stop, and put in enough drops of H2SO4 .1N to JUST bring it back clear again.
Add some more DI water, about 75mL is plenty, and put on your stir plate.
Add 40mL Boric acid solution, saturated (I use 60g in 1L hot DI water, shake and cool, it keeps for a long time) and 40mL reagent alcohol (ethanol-methanol mix).
Add 8g Potassium chloride. Allow everything to dissolve and mix well.
Add methyl orange indicator (my least favorite indicator ever!!) and titrate to full salmon-pink with H2SO4 0.1N.
mL H2SO4 0.1N x 4.5 =g/L ammonium bifluoride AS METEX ETCH SALTS... On the old MSDS, it's listed as 'Fluoride ion, 100% by weight' so you are absolutely going to want to make an analytical standard from fresh ammonium bifluoride, run a calibration curve, and confirm your multiplier.
Also, as the metals load up in the bath, it becomes wildly inaccurate... have fun! :)
There's also a titration listed for nitric acid in the presence of the Etch Salts that basically has a small correction [subtractive] for the interference of the salts themselves. Other than that, it's a normal Nitric test:
10mL bath sample, few drops methyl red indicator (you could probably get away with phenolpthalein for this just fine), titrate with 1.0N NaOH to yellow if using MR or pink if using PTH.
Without the ammonium bifluoride correction, it's mL NaOH 1N x 7.06 = %vv as 42'Be or 67% Nitric acid. With the Etch Salts correction, it's (mL NaOH1N -(oz/gal etch salts x .131)) x 7.06= %vv as HNO3 67%.
We use the etch salts without Nitric to frost Aluminum (2xxx, 6xxx, 7xxx) prior to desmutting and then anodizing. I would actually recommend just getting a sample of MacDermid's product and trying that out, and then if it gives the results you are looking for, you can use their calculations with greater confidence.
Good luck!

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.